Dataset: Microplastics photodegradation

Experimental

2.1 Seawater and general sample handling

All sample handling and experimental procedures followed trace clean oceanic protocols for DOC[29]. All plasticware was cleaned by triple rinsing with ultrapure water (MilliQ), soaking overnight in ~pH 2 water (4:1000, v:v, 6N HCl:MilliQ), triple rinsing with MilliQ and then drying. Glassware and quartzware were cleaned as above and then ashed at 450 °C for 6 hours to remove any trace organics. Seawater (salinity ~35) was collected from ~5 m in the South Atlantic Bight using Niskin bottles aboard the RV Savannah and gravity-filtered (0.2 μm; AcroPakTM 1500, PALL) directly from Niskin bottles into pre-cleaned 20 L high density PE carboys. In order to remove natural, photochemically active organics before adding plastics, seawater was transferred to 2 L ashed quartz flasks and placed under germicidal ultraviolet-C light until colored dissolved organic matter was undetectable using an Aqualog spectrofluorometer (HORIBA Scientific) and the DOC concentration was stable.

2.2 Plastic preparation

Plastic particles collected from the North Pacific Gyre were provided by Algalita Marine Research and Education and the 5 Gyres Institute. The real-world North Pacific Gyre sample contained plastic-fragments with an average size of 5.9±3.1 mm (Fig. S1 in Zhu et al., 2020), including sub-5 mm microplastics and other small plastic fragments. Thus, we refer to this sample as containing plastic-fragments rather than using the more precise, operational term microplastics that refers specifically to particles under 5 mm[2]. Plastic particles were sonicated in MilliQ water for 10 minutes and then soaked in 1% H2O2 for two hours, and then triple sonicated for 5 minutes in MilliQ water. Once clean, the plastics were air dried prior to further analysis and use in experiments.

Postconsumer plastics were PE (RejoiceTM shampoo bottle), PP (NIVEA® facial cleanser bottle) and EPS (disposable lunch box). Postconsumer plastics were cut into small pieces (3.04±0.87 mm; Fig. S1B-D). In addition, a standard PE granule (PEstd) was purchased (diameter: 2 mm; Goodfellow, USA). Prior to further analysis and experiments, microplastics were cleaned by sonification in MilliQ water and dried.

The surface area and volume of each sample type was estimated as described in the Supplementary Methods. Surface area to volume ratios (SA:V) were for each sample type were then calculated (Table1 in Zhu et al., 2020).

Density of each postconsumer microplastics was assessed by the addition of microplastics to MilliQ water and then either calcium or ethanol was added to adjust the solution density until the microplastics maintained in a position of neutral buoyancy for 20 minutes[30]. 1 mL of the resultant solution was weighed to determine solution density. Measured densities are reported in Table S1 of Zhu et al. (2020). The density of EPS could not be determined via this method due to the presence of embedded air pockets. Therefore, data (0.01-0.05 g cm-3) from the manufacturer are reported (https://insulationcorp.com/eps/).

2.3 Irradiations

All plastics were cleaned by sonification in MilliQ water and dried prior to experiments to simulate prior exposure to water as expected for plastics found at sea. Four hundred and eighty cleaned pieces of each polymer type were randomly selected, weighed (XP26 DeltaRange, Mettler Toledo, readability is 0.01 mg), and divided into two groups (240 particles per group). For the North Pacific Gyre sample, plastic-fragments were evenly separated into two groups based on mass (AB265 S/FACT, Mettler Toledo, readability is 0.01 mg). This yielded a total of twelve plastic aliquots: 2×PE, 2×PE standard, 2×EPS, 2×PP and 2×North Pacific Gyre. Prior to irradiation these aliquots were rinsed three times with MilliQ, three times with seawater and then transferred into the 2 L ashed and ultraviolet-C sterilized spherical quartz irradiation flasks with 1 L of pre-photobleached seawater (two flasks for each plastic type = 10 flasks). Two control flasks were filled with pre-photobleached seawater, without plastics resulting in a total of 12 flasks. Half of the flasks (i.e. one of each treatment) were wrapped in heavy duty aluminum foil to provide dark controls. All flasks were then placed inside a solar simulator.

Irradiations were conducted in a solar simulator system equipped with 12 UVA-340 bulbs (Q-Panel) which provides light with a spectral shape and flux approximating natural sunlight irradiance from 295 to 365 nm[31]. This wavelength range is responsible for the majority of environmental polymer photochemical reactions[32-34]. The integrated irradiance (14±0.7 W m-2) in the solar simulator was quantified using a spectroradiometer (OL 756, Optronic Laboratories) fitted with a quartz fiber optic cable and 2-inch diameter integrating sphere which was calibrated with a National Institute of Standards and Technology (NIST) standard lamp (OL752-10 irradiance standard)[34]. Twenty four hours irradiation under these conditions approximates one solar day of photochemical exposure in the subtropical ocean gyre surface waters[20] where microplastics accumulate. A side mounted fan maintained temperatures between 25°C and 30°C. The flasks were repositioned daily to average out potential spatial variation in the light flux under the solar simulator. Throughout the seawater incubations DOC was monitored providing a time-series of DOC release and accumulation. In detail, liquid samples were drawn from the flasks using ashed glass Pasteur pipettes. Duplicate DOC samples (~10 mL aliquots) per time point were collected into pre-combusted 24 mL glass vials at 0 d, 2 d, 5 d, 10 d, 14 d, 22 d, 35 d, 49 d, 68 d for the North Pacific Gyre sample and 0 d, 0.5 d, 1 d, 3 d, 6 d, 10 d, 15 d, 22 d, 35 d, 54 d for postconsumer and standard plastic samples. Samples for bacteria counts (1 mL aliquots in sterile Nalgene cryovials) were collected at 0 d, 1 d, 15 d and 54 d for postconsumer and standard microplastics and at 0 d, 2 d, 5 d, 10 d, 14 d, 22 d, 35 d, 49 d, 68 d for the North Pacific Gyre sample, fixed with 0.1 % glutaraldehyde, and frozen at -80 °C until analysis. The remaining volume of each sample for the next time point irradiation was determined based on weight. Plastics were recovered from the flasks by filtering through 0.22 µm filters (GVWP04700, Millipore). After 54 days, fragments were dried, weighed and further analyzed for carbon content, and via optical and electron microscopy, and Fourier transform infrared (FTIR) spectroscopy.

2.7 Dissolved organic carbon (DOC)  

Samples for DOC analysis were acidified to pH < 2 using HCl (p.a.) before analysis using a Shimadzu TOC-VCPH total organic carbon analyzer[29]. Certified DOC standards (low carbon seawater, LSW and deep seawater reference material, DSR) from the Consensus Reference Materials (CRM, University of Miami) were measured to confirm precision and accuracy. Measured DSR values were consistent with the consensus value (0.49-0.53 mg L-1, http://yyy.rsmas.miami.edu/groups/biogeochem/Table1.htm) with a standard deviation <5%. Routine minimum DOC detection limits using the above configuration are 0.034±0.0036 mg L-1 and standard errors are typically 1.7±0.5 % of the DOC concentration[29].

Plastic carbon mass normalized DOC accumulation was calculated following Equation 1 (Zhu et al., 2020).

Where, PDOC is the accumulation of DOC produced during the photo-dissolution of microplastics; n is the total number of samplings; i is each time point; V and C are the sample volume and concentration at each time point, respectively, and; MC initial denoted the total plastic carbon mass (g) of plastic particles at the start of each experiment. Measurement uncertainties were propagated to calculate final errors[38].

Note about locations:

The seawater was from the Southern Atlantic Bight but the location is not relevant to the interpretation or use of this dataset. 

The plastics were collected from the North Pacific Gyre by non-profits (Algalita and 5 Gyres) studying the area.  The exact sampling locations are unknown. They were provided as representative samples.